This invention relates to disc brakes and more particularly to the elimination of "brake squeal" associated with motor vehicle disc brakes upon application of the braking force.
Conventional motor vehicle disc brake systems include a rotatable disc or rotor mounted on the wheel and an nonrotatable member or caliper secured to a support member. The caliper carries a pair of spaced brake pads having load bearing faces of a high friction type of material on either side of the rotor which is sandwiched therebetween. The facing of the pads is bonded, rivoted or otherwise secured to a rigid metal base. The caliper includes a housing within which is mounted a piston operable by hydraulic fluid. The piston thrusts against the rear or base side of the inboard pad when the brake is applied to force the facing of the inboard pad against the interior face of the rotor, and the reaction forces on the caliper force the caliper to pull the outboard pad against the other face of the rotor.
A longtime problem associated with disc brakes of this design is the noise or "squeal" that occurs when the brakes are applied. The problem has been recognized by the prior art (see for example U.S. Pat. Nos. 4,075,142; 3,887,044; 3,722,634 and 3,378,116) to eminate from vibrations, whether coupled or not, of the pads, caliper and/or disc. Not only do these vibrations create an annoying noise, but the efficiency of the braking system is reduced by the high frequency oscillatory movement of the pads when the brakes are applied resulting in longer stopping distances than would result with an efficient system. Another effect is that the vibration energy is transmitted into heat energy and since heat reduces brake life and results in brake fading the life of the brakes is less than would otherwise be possible.
Proposed solutions to this problem abound in the art. In U.S. Pat. No. 4,075,142 a chemical coating including a polymeric emulsion is applied to the back of the outboard pad. A spray solution of a material presumably of this type is on the market and provides a temporary fix. In U.S. Pat. No. 3,887,044 an elastic ring is positioned in a radial groove in a bolt adjacent one end and inserted into a bore in the piston with the other end secured to the inboard pad, while the outboard pad has one end of a bolt secured thereto and the other end has a radial groove inserted into a spring secured to the caliper. It thus requires extensive rework, and redesign of the brake system. In U.S. Pat. No. 3,378,116 a disc of resiliant material is bonded to the back of the pad base member, and the solid face piston engages the disc when the brake is actuated. The piston must act against the pad if this system is to be effective. Since pistons today are hollow with a thin wall shell, this construction is ineffective. Moreover, it would require a piston for the outboard pad, which is not today conventional.